The present invention relates to semiconductor light emitting devices and processes for producing same. More particularly, the present invention relates to a semiconductor light emitting device fabricated by forming a growth layer having a stacked structure of a first conductive layer, a light emission layer, and a second conductive layer by selective growth on a growth substrate, and a method of fabricating the semiconductor light emitting device, the semiconductor light emitting device being fabricated by forming a wurtzite type compound semiconductor layer such as a gallium nitride based compound semiconductor layer by selective growth.
Conventionally, when manufacturing a semiconductor light emitting device of this type, a device is fabricated by forming a low temperature buffer layer overall on a sapphire substrate, forming an n-side contact layer made from Si-doped GaN thereon, and stacking, on the n-side contact layer, an n-side cladding layer made from Si-doped GaN, an active layer made from Si-doped InGaN, a p-side cladding layer made from Mg-doped AlGaN, and a p-side contact layer made from Mg-doped GaN. As commercial products of semiconductor light emitting devices having such a structure, light emitting diodes and semiconductor lasers allowing emission of light of blue and green in a wavelength ranging from 450 nm to 530 nm have been fabricated on a large scale.
A sapphire substrate has been often used for growing gallium nitride thereon. However, dislocations may occur in crystal, at a high density, due to mismatches between crystal lattices of the sapphire substrate and gallium nitride. A method of forming a low temperature buffer layer on a substrate is one way of suppressing such defects occurring in crystal during growth thereof. In a method disclosed in Japanese Patent Laid-open No. Hei 10-312971, usual crystal growth is combined with selective crystal growth in the lateral direction (ELO: Epitaxial Lateral Overgrowth) for reducing crystal defects. The method of fabricating a semiconductor light emitting device disclosed in Japanese Patent Laid-open No. Hei 10-312971 has also described that through-dislocations propagated in the direction perpendicular to a principal plane of a substrate are bent in the lateral direction by a facet structure formed in a growth region during fabrication and are thereby prevented from being further propagated, thereby reducing crystal defects.
On the other hand, there has been known a method of fabricating a semiconductor light emitting device in a fine region by forming a layer of a nitride based semiconductor such as GaN into a pyramid shape by selective growth. In particular, a method of fabricating a light emitting device by forming a hexagonal pyramid shaped nitride based semiconductor layer by selective growth has been disclosed, for example, in xe2x80x9cSpatial Control of InGaN Luminescence by MOCVD Selective Epitaxy, D. Kapolnek et al., Journal of Crystal Growth, 189/190 (1998) 83-86xe2x80x9d. According to the selective growth technique described in this document, a plurality of nitride based semiconductor light emitting devices, each of which is composed of a fine hexagonal pyramid shaped GaN/InGaN layer structure, can be formed. With respect to such a fine hexagonal pyramid shaped light emitting device, it has been known that an active layer is formed on an S-plane (i.e., a (1-101) plane) formed in self-alignment, thereby improving crystallinity and light emergence efficiency.
When forming a light emitting device composed of a hexagonal pyramid shaped nitride based semiconductor layer having a stacked structure by selective growth, a p-side electrode and an n-side electrode are required to be formed on a selectively grown stacked layer for supplying a current to a light emission layer. In general, at the time of selective growth, a p-side conductive layer is stacked on an inside conductive layer. Accordingly, to form both n-side and p-side electrodes, part of the p-side conductive layer must be removed by etching or the like. To be more specific, an n-side electrode is typically formed by forming an n-type first growth layer, forming a growth obstruction film for selective growth on the first growth layer, forming a second growth layer by selective growth, forming a window in the growth obstruction film at a position where the second growth layer is not formed, and forming the n-side electrode in the window.
FIGS. 4A and 4B are views showing a hexagonal pyramid shaped semiconductor light emitting device formed by typical selective growth. As shown in FIG. 4A, a first growth layer 81 made from GaN or AlN is formed on a sapphire substrate 80, and a growth obstruction film 82 made from silicon oxide or silicon nitride is formed on the first growth layer 81. Subsequently, in each device region, an opening portion 83 is formed in the growth obstruction film 82, and a second growth layer is formed by selective growth from the opening portion 83. The second growth layer has a stacked structure of an n-type first conductive layer 84, an active layer 85, and a p-type second conductive layer 86.
The second growth layer is a hexagonal pyramid shaped growth layer, and a p-side electrode 87 is formed on the second conductive layer 86 as the outermost portion of the second growth layer. On the other hand, in each device region, a window 89 is formed in the growth obstruction film 82, and an n-side electrode 88 is formed in the window 89. After formation of the n-side electrodes 88 and the p-side electrodes 87, as shown in FIG. 4B, device isolation for isolating light emitting devices from each other is performed. To be electrically connected to the n-side electrodes 88, the first growth layer 81 positioned under the growth obstruction film 82 is doped with an n-type impurity. Such a conductive first growth layer 81 is required to be divided into parts belonging to respective device regions. The device isolation is generally preformed by forming device isolation trenches 90 by etching. A principal plane of the sapphire substrate 80 is exposed at bottoms of the device isolation trenches 90.
When fabricating light emitting devices by forming growth layer portions each having a hexagonal pyramid or a truncated shape thereof, or another pyramid shape or a truncated shape thereof by selective growth and independently driving respective devices or transferring or mounting respective devices on another substrate, the first growth layer 81 as an under growth layer must be isolated into parts belonging to respective device regions.
In this case, however, since the second growth layer is formed into a hexagonal pyramid shape or another pyramid shape by selective growth from the opening portion formed in the growth obstruction film at a position in each device region, there is a relatively large difference-in-height between a top portion of the pyramid shaped second growth layer and the surface of the growth obstruction layer. In particular, the surface portion of the growth obstruction film becomes the recessed side of the difference-in-height. As a result, the device isolation trenches 90 for isolating the devices from each other must be formed in the recessed regions by etching. Because of the difference-in-height between the top portion of the second growth layer and the surface of the growth obstruction film 82, it is not easy to form the device isolation trenches 90 with desirable repeatability, and in the worst case, device isolation becomes impossible due to positional deviation of a mask for forming the device isolation trenches.
An object of the present invention is, therefore, to provide a semiconductor light emitting device and a method of fabricating the semiconductor light emitting device, which are capable of isolating respective devices from each other with desirable repeatability.
According to an embodiment of the present invention, a semiconductor light emitting device is provided. The device includes a growth substrate, a first growth layer formed on the growth substrate, a growth obstruction film formed on the first growth layer; and a second growth layer formed by selective growth from an opening portion formed in the growth obstruction film. The second growth layer has a stacked structure of a first conductive layer, a light emission layer, and a second conductive layer. The device further includes device isolation trenches for isolating devices from each other. The trenches are formed in the first growth layer formed on the growth substrate. Preferably, the second growth layer is formed by selective growth after formation of the device isolation trenches.
With this configuration, since the device isolation trenches for isolating respective devices from each other are formed in the first growth layer, the first growth layer can be electrically isolated into parts belonging to respective device regions. In this regard, the device isolation trenches are preferably formed before the formation of the growth layer by selective growth. Accordingly, at the time of formation of the device isolation trenches, the pyramid or polygonal shaped growth layer is not yet formed. Thus, irregularities on the substrate are small. Accordingly, the device isolation trenches can be formed with desirable repeatability.
According to another embodiment of the present invention, a method of fabricating a semiconductor light emitting device is provided. The method includes the steps of forming a first growth layer on a growth substrate, forming device isolation trenches for isolating devices from each other in the first growth layer, forming a growth obstruction film having a specific opening portion in the first growth layer in which the device isolation trenches have been formed, and forming a second growth layer by selective growth from the opening portion. Preferably, the second growth layer has a stacked structure of a first conductive layer, a light emission layer, and a second conductive layer.
With this configuration, the selective growth step is performed after the step of forming the device isolation trenches for isolating respective devices from each other. Accordingly, at the time of formation of the device isolation trenches, the growth layer is not yet formed. Thus, the device isolation trenches can be formed with good or desirable repeatability. Since the growth obstruction film on the first growth layer is formed after the formation of the device isolation trenches, the growth obstruction film is formed even on side walls of the device isolation trenches. As a result, since a surface area of the growth obstruction film on the substrate becomes large, it is possible to supply a larger amount of source gases required for selective growth onto the opening portions formed in the growth obstruction film.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.